The present invention relates to a process for controlling a crust breaking facility having a chisel which can be moved up and down to penetrate the crust on top of the molten electrolyte in a fused salt electrolytic cell, and this by detection of the contact made between the chisel and the molten electrolyte, such that changes in signal caused by changes in impedance between chisel and molten electrolyte are detected for control purposes using the chisel as a sensor in an electric measuring circuit, and relates too to a device for controlling a crust breaking facility having a chisel which can be moved up and down to penetrate the crust on top of the molten electrolyte in a fused salt electrolytic cell such that control is made via detection of the contact made between the chisel and the molten electrolyte by means of an electrical measuring circuit with the chisel as a measuring sensor, in which circuit the chisel/molten electrolyte path appears as an impedance element indicating contact.
In the production of aluminum by fused salt electrolysis of aluminum oxide the latter is dissolved in a fluoride melt comprising, for the greater part, of cryolite. The cathodically precipitated aluminum collects on the carbon floor of the cell under the fluoride melt, the surface of the liquid aluminum itself forming the cathode. Dipping into the melt from above are anodes which in the conventional process are made of amorphous carbon. At the carbon anode oxygen is formed as a result of the electrolytic decomposition of the aluminum oxide; this oxygen combines with the carbon of the anodes to form CO.sub.2 and CO. The electrolytic process takes place in a temperature range of about 940.degree. to 970.degree. C.
In the course of the reduction process the aluminum oxide i.e. the alumina in the electrolyte is consumed. At a lower concentration of about 1-2 wt.% alumina in the electrolyte the anode effect occurs, which produces an increase in the voltage from for example 4-5 V to 30 V and higher. The cell is therefore usually serviced periodically during normal operation, even when no anode effect occurs. In addition, every time the anode effect occurs the alumina concentration in the electrolyte must be raised by feeding aluminum oxide to the cell.
In the case of hooded cells maximum retention of the cell fumes in the system is obtained if the feeding of the cell takes place automatically at brief intervals. Both the now conventional, local and continuous pointfeeder principle and the discontinuous feeding of alumina along the whole longitudinal or transverse axis of the cell can be employed.
The storage bunkers or alumina silos situated on the reduction cells are generally in the form of funnels or containers with a funnel or conical-shaped lower outlet. The contents of the silos mounted on the cell are usually adequate to supply the cell for one or two days. The silo is therefore also known as a day's supply silo. Up to now the supplying of such a silo with alumina was usually via a closed pipe system, preferably with compact flow feeding from the central alumina supply.
The feeding of the alumina from the day's supply silo to a break in the crust covering the molten electrolyte is usually performed via known devices whereby a flap is swung open for charging purposes, or in another system via feeding screws, measured feed cylinders or measured volumes.
Another device for feeding alumina is such that there is no day's supply silo on the cell, and the measured feeding device is situated away from the cell.
An essential feature of continuous feeding of alumina is that the opening in the crust is always kept open so that the alumina can be fed in measured quantities to the electrolyte. In modern electrolytic cells therefore the alumina feeding and crust breaking facility are always spacially and functionally combined. An electronic process control signal first initiates the raising and lowering of the chisel of the crust breaker, immediately after which the feeding of the alumina takes place.
A mechanically or pneumatically actuated end switch stops the lowering action of the chisel and causes the chisel to return to the resting position. As a result the chisel remains for a period of time in the molten electrolyte where it corrodes relatively quickly and consequently has to be replaced prematurely. Furthermore, crust material remains stuck to the strongly heated chisel and must be wiped off. The amount of compressed air consumed is relatively high.
Known from the French patent publication FR-PS 2 483 965 is a process and a device of the kind mentioned at the start, in which the chisel is used as a measuring sensor in an electric measuring circuit which runs from the chisel to the cathode of the cell via a recording instrument. If the chisel dips into the molten electrolyte, then the direct voltage produced by the electrolytic d.c. current between the molten electrolyte and the cell cathode appears on the recording instrument as a signal that the chisel has made contact with the molten electrolyte and serves as a criterium for control purposes.
Trials have now shown that satisfactory results in the form of an unambiguous criterium for control cannot be obtained with this method. An explanation for this is as follows:
The anode/cathode voltage which or part of which is tapped off by the process according to this French patent publication is subject to fluctuations. As is generally known such molten salt reduction cells are fed with electric current from a single source, a plurality of such cells being connected in series. Consequently the anode/cathode voltages cross the individual cells depend on the resistances prevailing between the anodes and the cathodes, and are not kept constant by a fixed voltage supply. As already mentioned above, the anode/cathode voltage can fluctuate within a large range especially when the anode effect occurs. Such, usually unpredictable fluctuations, which are generally of a magnitude that they disturb the anode/cathode voltage of the cell, affect the measurement if the known method is employed as with that method one measures the voltage of cell operation parameters.